The present invention relates to an apparatus for monitoring and controlling the operation of a distillation column. More specifically, the invention relates to an apparatus for division of a liquid stream within the column into a side stream and a reflux of metered and controlled flowrate.
Separation of a fluid mixture into various component fractions according to relative boiling points or boiling point ranges may be accomplished by fractional distillation, generally carried out in a vertical column containing trays or beds of packing material which effect intimate contact between the vapor flowing up through the column and the liquid flowing down. Such distillation columns yield an overhead product consisting of a lower boiling fraction and a bottoms product of a higher boiling fraction. In addition, it is often desired to remove one or more intermediate boiling fractions as side streams from intermediate locations in the column. When a liquid side stream withdrawal is made, it usually comprises only a part of the liquid flow in the tower at the point of withdrawal. In this situation of partial withdrawal, the remainder of the liquid passes to the fractionation stages of the column below the side stream draw as a reflux to maintain liquid-vapor contact and effect efficient fractionation in this portion of the column.
In order to take such a partial side stream draw, it is necessary to accomplish a division of a liquid flow within the column into a side stream and a reflux. Since the reflux liquid remains within the distillation process and influences its operation, it is often advantageous to make this division in a manner which allows both measurement and control of reflux flowrate. Additionally, it is most desirable that the measurement and control functions be carried out in a manner such that no more of a hydraulic driving force is required than that which is normally available for gravity flow of liquid from the point of side stream draw to the next lower fractionation stage. Operating within the constraints of gravity flow obviates the need for the pumping of reflux flow through metering and control devices, a common conventional practice.
At least one such device has been proposed for use in dividing a liquid flow into side stream and reflux which also accomplishes metering of the reflux flowrate. The invention disclosed in U.S. Pat. No. 2,254,182 to W. W. Kraft is directed to an apparatus comprising a chamber for collecting the total liquid traffic down the column at the point of the desired partial side stream draw. Reflux and side stream flow continuously out of the chamber, each through one of two outlets. The reflux outlet consists of a calibrated orifice located on the bottom of the chamber. Since flow through the orifice is dependent in a known manner upon liquid head above the orifice, monitoring of liquid level in the chamber provides indirect measurement of reflux flowrate.
It is further known that the concept of flow measurement illustrated by U.S. Pat. No. 2,254,182 can be adapted to provide some degree of reflux flow control. For instance, if a given constant flow of reflux is desired, the side stream flowrate could, in theory, be varied as necessary to provide a given constant liquid level in the chamber and thus a constant flow through the chamber orifice. However, such indirect control logic has, in practice, proven unsuccessful for providing stable control of reflux flow for the reason that liquid traffic through a distillation column is generally characterized by continual fluctuations in flowrate. Such fluctuations are variable, both as to amplitude and frequency, in a manner which does not permit a response of side stream control which is adequate to prevent these fluctuations from being reflected in the chamber liquid level and hence in the reflux flowrate. In essence, the particular control logic fails in this application because it is unable to reproduce the variations in liquid traffic with identical corresponding variations in side stream flow. As a consequence, minor upsets and normal variations in the distillation process above the side draw tray are communicated to the lower sections of the column through variations in reflux flow.
An additional shortcoming of many conventional metering chamber devices is their inability to adapt to substantial changes in the operation of the column. Both metering and control functions of prior art devices are accomplished by the same orifice, calibrated for operation under a given mode of column operation. Over the long term, major changes in the distillation process, involving, for instance, variation in feed flowrate, feed composition, or side stream flowrate, will necessitate a shutdown of column operations for replacement of the orifice in order that it will function adequately in the new mode of operation.